The Financial Industry Regulatory Authority (FINRA), one of the bodies that regulated the company, became concerned that MF Global had a substantial position in European sovereign debt in June and was not appropriately holding capital against it, according to reports in the US.

FINRA, which regulates the securities-brokerage arm of MF Global, opened dialogue with the company about whether it was appropriate for it to use Generally Accepted Accounting Principles (GAAP) to park the exposure off balance sheet.

MF Global, which has almost 3,000 staff on either side of the Atlantic, resorted to bankruptcy protection after its $6.3bn bet on European government debt drained the confidence of shareholders and customers. Jon Corzine, MF Global's chief executive, pushed MF Global to take more risk as part of a plan to turn the broker into an investment bank.

Federal Reserve chairman Ben Bernanke has distanced the US government from MF Global's collapse. In February, the Fed made MF Global a primary dealer - one of the 22 institutions that help the US government sell its bonds.

Speaking on Wednesday, Mr Bernanke said: "We're not the regulator of MF Global - that's the SEC and the CFTC. Making them a primary dealer did not in any way constitute a seal of endorsement.

"It appears to be an idiosyncratic case. We've been monitoring the impact on certain funding markets and so far there's little impact on financial stability."

US regulators' estimate of the shortfall in MF Global's accounts has declined but it is still at several hundred million dollars, according to sources.

"The company originally disclosed a $900m shortfall and that's going to be moving around as money comes back into these accounts," one said.

"It's dropped some, but it's going to be a fluid thing. We do know there was a shortfall ... and that is where things stand. We don't know why there was a shortfall or where it went."

MF Global's collapse made it the biggest US casualty of Europe's debt crisis, and the seventh-largest bankruptcy by assets in US history.

The investigation is continuing although regulators have not been able to determine if customer funds were used for the firm's own financial needs.

On Tuesday, CME Group, which owns the Chicago Mercantile Exchange and acts as a self-regulator for companies that trade on it, said MF Global failed to separate its customers' money from its own trading accounts.

"While we are unable to determine the precise scope of the firm's violation at this time, we are investigating the circumstances of the firm's failure," said Craig Donahue, chief executive of the CME, which audits the position of firms who trade on the exchange.